Not Applicable.
1. Field of the Invention
This invention relates to agriculture, and more particularly to a method and apparatus for determining a need for fertilization in a crop and applying the fertilizer in response to that measurement in real time.
2. Description of the Related Art
The strong relationship between nitrogen availability in soil and crop yields is well known in the art. Of the major plant nutrients (N, P, K, Ca, Mg, and S), nitrogen is the nutrient required in the greatest quantities and much research has been conducted over the years to determine the nitrogen fertilizer requirements of crops. Applying too little nitrogen causes severe economic losses by reducing crop yields, while applying too much nitrogen is not only expensive but also causes harmful levels of NO3 to leach into the ground and surface water supplies.
Determining the need for nitrogen is particularly difficult since most soil nitrogen is tied up in the soil organic matter and is unavailable to plants. The amount of nitrogen that is made available (in the form of NO3 and NH4) to plants during the growing season depends strongly on factors such as the previous cropping history of the field, soil organic matter content, microbial activity and rainfall (which quickly leaches the available forms of nitrogen away from the root zone). These difficulties have led some investigators to explore the use of plant nitrogen analysis instead of soil tests, since the plant itself should be a reliable indicator of the amount of nitrogen available to it.
Chemical assaying of plant nitrogen status is typically a slow and time consuming process because, as in the case of corn, about a dozen leaves are usually collected which must then be dried, ground, and analyzed using one of several laboratory techniques. Interpreting the laboratory data involves considerable uncertainty because leaf nitrogen concentrations decrease during the growing season and nitrogen concentrations further depend on which leaf is sampled. These difficulties led to efforts to develop rapid, non-destructive methods for assessing the nitrogen status of crops.
Crops that are well-supplied with nitrogen grow vigorously and look deep green in color due to their high leaf chlorophyll concentrations, while crops deficient in nitrogen grow less well and look yellow-green (chlorotic) due to low concentrations of chlorophyll. This led to the development of a device for measuring leaf chlorophyll content, called a chlorophyll meter, which measures light transmittance through individual leaves in the red (approximately 670 nm) and near-infrared (approximately 800 nm) regions of the electromagnetic spectrum. Furthermore, above a certain level of nitrogen fertilizer application, crop growth and chlorophyll content reach a plateau or a maximum, referred to hereinafter as the nitrogen fertilizer response plateau. This permits the use of a reference strip which provides a baseline against which nitrogen stress can be measured. However, due to the fact that a very great number of leaf measurements must be taken in order to survey an entire field for variable rate fertilizer adjustments, this method has not been widely accepted in commercial agriculture.
An additional problem with leaf chlorophyll or leaf color measurements is that the difference in leaf chlorophyll (or color) between a nitrogen deficient plant and a nitrogen sufficient plant is relatively small. By the time nitrogen fertilizer deficiency has caused a detectable change in leaf color, significant and unrecoverable yield reductions have already taken place. Therefore, leaf chlorophyll measurements using leaf transmittance have to be very precise, requiring measurements to be done under controlled illumination and light-path geometry. Measurements must be done on individual leaves clamped within an instrument. It is not possible at present to measure the entire crop canopy chemical attributes (color, chlorophyll or nitrogen concentration) with sufficient precision to be of value for detecting nitrogen deficiencies before irretrievable losses have already taken place. Measuring entire crop canopy reflectance in the green band does not measure leaf greenness (i.e. color), it merely measures the reflection in the green band of contrasting targets within the field of view of the sensor (plants, soil and shadows). Using chlorophyll fluorescence is even more problematic because fluorescence is not only a function of nitrogen status and leaf chlorophyll content but also depends on the short-term changes in the plant""s metabolic pathways. Fluorescence is a very unstable physiological parameter which varies from hour to hour with illumination, drought and a variety of other stresses. It is difficult to see how it could be used for determining the nitrogen needs of a field crop.
A more sensitive alternative to measuring color change is to measure changes in standing crop biomass or some other physical attribute such as leaf area, crop density, crop cover, etc. Measuring the entire crop canopy physical attributes is radically different from and has several advantages over measuring individual leaf chemical attributes such as chlorophyll content or color. Changes in standing crop density, for example, show a greater sensitivity to nitrogen supply than do changes in leaf color. Also, the measurement is integrated over a larger area and is much easier to do in real time. It can, for example, be done from a tractor or center pivot using one of several rapid, non-destructive methods or from aircraft or satellites using crop reflectance.
Various rapid, non-destructive techniques for assessing standing crop biomass and other related canopy physical attributes such as leaf area index and percent ground cover have also been developed. Included among these is the measurement of canopy reflectance in the visible, near-infrared (NIR), and scattering in microwave regions of the electromagnetic spectrum. Other techniques for rapid, non-destructive estimation of crop standing biomass have included measurements of canopy electrical capacitance, attenuation of xcex2-particles and other ionizing radiation, and measurements of crop canopy transmittance in certain regions of the solar spectrum (between 300 nm and 3000 nm). The use of acoustic energy for estimating the growth response of crops to fertilizer has not been reported in the literature. However, acoustic techniques have been used for a variety of other rapid, non-destructive measurements such as determining animal back-fat and pregnancy detection. Some of these methods have certain advantages over the present optical embodiment because they do not rely on sunlight. Recent tests by the inventors suggest that acoustic measurements can provide the canopy information necessary for determining crop nitrogen requirements.
Standing alone, these measurements of entire crop canopy physical attributes have little commercial value. Also, the measurements are only relative and have values that depend on the conditions of measurement such as crop growth stage, canopy structure, and environmental conditions. However, combining one of the measures of crop density in real time with the use of a reference strip provides a relative measurement that is sufficient to determine crop nitrogen status and the need for fertilizer in real time.
The present invention discloses a method and apparatus for the real time determination and application of optimum amounts of nitrogen fertilizer to corn and other arable crops such as cotton, sugar beets, wheat, etc. by the use of rapid, non-destructive crop canopy sensors and various fertilizer application methods. In a first embodiment, optical reflectance measurements of the crop canopy and a reference strip at the fertilizer response plateau are taken by sensors carried by a center pivot irrigation system. A sensor controller receives, analyzes and stores the sensor data as well as position data received from the center pivot controller. The sensor controller then sends commands to the center pivot controller to order fertilizer application, as necessary, through the irrigation system.
A second embodiment of the invention utilizes a tractor drawn fertilizer applicator with the sensors mounted on the application boom. As in the first embodiment, a sensor controller receives, analyzes and stores reference strip and crop canopy reflectance data and sends commands to the tractor fertilizer controller to order fertilizer application. This second embodiment may also utilize the Global Positioning System to provide tractor position for storage along with crop data for later use and comparison.
The location and size of the reference strips will depend on the particular embodiment in use. In the first embodiment, the reference strip would preferably consist of a ring of crop from which a particular sensor could continually receive data as the irrigation system pivots around the field. In the second embodiment, a series of reference strips may extend across the field for continual monitoring, or a reference area may provide reference data which would then be memorized by the sensor controller. The establishment of the reference strips will depend to a large extent on convenience and preference of the farmer.